The invention relates to microwave systems operating in the submillimeter wave range, typically from 10 .mu.m to 1 mm, suitable for use as sources, detectors and mixers.
There exist some high-speed broad band detectors suitable for use in the submillimeter range, which belong to the far infra-red and the microwave field. Almost all of these detectors use non-linear elements consisting of Schottky diodes which have the advantages of wide band characteristics and operation at ambient temperature. However, the Schottky diodes must then have a very small surface (less than a square micron) and they must be associated with a feed structure. The high losses in materials conductive to submillimeter waves prevent using metal wave guides and lead to using antennas for coupling the waves.
In a prior art detector, the supply structure for coupling the incident radiation and the Schottky junction comprises a wire whose pointed end portion contacts the anode of the diode. The antenna consists of the portion of the wire between the contact point and a highly curved portion which also acts as a spring forcibly applying the wire against the anode. Such a coupling between the long antenna thus formed and the non-linear element constituted by the Schottky diode is anything but satisfactory. The antenna has a radiation pattern with several transmission and reception lobes and the radiation diagram has a rotational symmetry. Consequently, part only of the main lobe is irradiated by the incident radiation very directional at that frequency; sensitivity is consequently decreased and in addition part of the energy is re-transmitted to lobes other than that which collects microwaves energy.
It is an object of the invention to provide an improved system suitable for use in the submillimeter range, which has a high sensitivity whilst simple in construction.
For that purpose, the system comprises a plurality of non-linear elements distributed according to a planar one or two dimensional array and an antenna structure for coupling said plurality of elements and the submillimeter wave radiation. The structure includes a curtain of long antenna wire antenna elements, i.e., antenna elements several wavelengths long, in parallel electric relation and having point-like contacts with said plurality of elements. A point-like contact will designate a contact between an electrode of the non-linear element and part only of the antenna over an area which has a size which is a fraction of the wavelength, for instance 10 .mu.m if the wavelength is 300 .mu.m. In this respect it will be appreciated that the size of a Schottky diode may be as low as 0.1 .mu.m.
When the system is to be used as a detector or mixer, the non-linear response elements will typically consist of a two dimensional array of Schottky diodes on a composite semiconductor plate, such as gallium arsenide. Provided that epitaxial doped gallium arsenide is used, so that the conduction is by a thermo-ionic process rather than by tunnel effect, it is possible to achieve satisfactory operation up to a frequency of about 3 THz. Other non linear elements may however be used, such as MIM (metal-insulating material) and planar Josephson junctions.
The increase in sensitivity provided by the invention does not result merely from the increase in the number of associated detector and antenna elements, but essentially from the radical change of the radiation pattern due to elimination of the rotational symmetry and enhancement of a main antenna lobe at the expense of the others. In use the detector will be angularly located for the direction of the incident radiation to coincide approximately with the axis of the main lobe.
When the system is to be used as a source, the non-linear elements may be Schottky diodes with tuning on a higher harmonic, whereby it is possible to reach 800 MHz. Tunnel diodes and quasi-particle junctions operating at cryogenic temperatures may also be used.
Numerous embodiments of the flat antenna structure are possible. The array should be usch that the antenna elements are correctly connected to the anodes of the diodes, are perfectly parallel and are evenly spaced where they contact the diodes.
In a particular embodiment, the curtain comprises a set of parallel wires each in contact with an associated non-linear element through a blunt pointed tip; but it is then difficult to maintain satisfactory parallelism of the wires without burying them in a dielectric support. Now, the majority of dielectrics commercially utilized at present are absorbent over a wide band of the spectrum, whence loss of sensitivity. In a modified embodiment, the set of wires are in the form of thin linear conductors constituted by photoprinting on an insulating layer which also has a superstrate of semi-conductor material in which the non-linear response elements, typically Schottky diodes, are constituted. Such a system is of advantage since it may be manufactured by conventional planar technology used in the LSI field. In yet another embodiment, the curtain comprises an electrically conductive plate (which amounts to an infinity of close wires), coupled to the non-linear response elements by separate pointed tips or cat whiskers. Experience has shown that good results are obtained with a flat plate whose length l corresponds to a few wave lengths and whose width W is such that 3.5/5.ltoreq.w/l.ltoreq.4/5. The pointed tips may be the end portions of parallel wires electrically and mechanically fast with the plate, arranged symmetrically with respect to the mid plane of the latter, two end wires being arranged along the lateral edges.
Finally, to further reduce the secondary lobes and reinforce the main lobe, it will generally be advantageous to associate one or several reflectors with the antenna curtain. Several reflectors may be combined with an input grid to form a tuned resonant cavity.
Whatever the embodiment, broad band detection may be achieved; associating a feeding antenna structure to a plurality of non-linear elements facilitates mutual matching. The invention may be implemented using planar techniques.
The invention is suitable for use in numerous applications, particularly the construction of submillimeter video-frequency radiation detectors and of mixers supplying an intermediate frequency resulting from bearing of the submillimeter waves and of a harmonic from a local source, such as a phase referenced klystron. The radiation may originate from various sources including lasers, for example hydrocyanic acid lasers emitting at 0.80 THz, carcinotrons and gyrotrons which have the advantage of supplying a very high power.
The invention will be better understood from the following description of particular embodiments.